| Soil acidification is a main form of soil degradation and a main factor that limits the farmland fertility. Under the laboratory conditions, Plinthudult and Paleudalfs from Zhejiang Province were incubated with different kinds of nitrogen (N) fertilizers (urea and ammonium sulfate) and leached with simulated acid rain (SAR). The aim was to study the effect of urea (U) and ammonium sulfate (AS) on soil acidification, to study the combined effect of N fertilizer and SAR on the cation leaching and aluminum (Al) dissolvd and on the soil pH, exchangeable hydrogen ion (H+) and aluminum ion (Al3+), soil buffering capacity (pHBC), soil organic matter (SOM) and soil sicrobial community. The main results were showed as follows:The results showed that the application of N fertilizers resulted in soil acidification. Urea caused greater acidification of the Plinthudult soil than ammonium sulfate, while there were no differences between these two treatments on the acidification of the Paleudalfs. When urea was applied to soil, first hydrolyzed along with soil pH increase, then nitrification began along with soil pH decrease; once ammonium sulfate was applied to soil, nitrification began and soil pH decreased. The dynamics of exchangeable H+and Al3+corresponded to that of soil pH. Correlation analysis showed that soil exchangeable H+and Al3+were significantly related with soil pH, respectively. At the end of the trial, the pHBC of Plinthudult in AS treatment was greater than that in CK and U treatments, which may be due to the buffering system of N+4+and NH4OH. However, the pHBC of Paleudalfs was unchanged between treatments.An unfertilized bulk control sample (CK) and a similar soil sample given N (urea, N200mg kg-1soil) were incubated for30days. Then the leaching experiment began. Six treatments were tested:SAR of pH2.5,4.0and5.6leaching on unfertilized soil (Treatments AR2.5, AR4.0and AR5.6), and on fertilized soil with urea (Treatments NAR2.5, NAR40. and NAR5.6). Severe acidic input increased Ca, Mg and K leaching and Al dissolvd in both unfertilized and fertilized soils. N application increased the intial leaching of Ca and Mg leaching in Paleudalfs and the total amout of Al dissolvd, but did not increase the total amout of Ca, Mg and K leaching. As one of the most mobile cations in soil, there was almost no difference between Na concentrations leached under the different treatments. The cumulative concentrations of dissolved organic carbon (DOC) leached under treatments AR2.5and NAR2.5were less than that under the other four treatments.After leaching, the order of the pH in Plinthudult was NAR2.5<AR2.5<AR4.0, AR5.6, NAR4.0and NAR5.6, and NAR2.5<AR2.5<NAR4.o, NAR5.6<AR4.0, AR5.6for Paleudalfs. Both N application and SAR had a significant effect on soil pH. The application of N fertilizers increased the exchangeable H+and Al3+in soil and SAR introduced large amout of H+and speeded Al leaching. N fertilizer and SAR, as well as the interactions between them, had a significant impact on exchangeable H+and Al3+. After being treated with SAR of pH2.5, soil entered the secondly buffer system, and soil was still in primary buffer system under the other four treatments. Treatments AR4.0, AR5.6and NAR5.6increased the pHBC of Plinthudult and treatments AR5.6and NAR5.6increased the pHBC of Paleudalfs. SOM of these two soils under different treatmens was of no difference, but were lower than that of original soil.Futher investigations were conducted by using phospholipid fatty acids (PLFAs) to follow the soil microbial community changes under N applition and SAR. N fertilizer and SAR had no effect on Fungi and actinomycetes in Plinthudult. Severe SAR (pH2.5) inhibited the growth of Gram-positive and Gram-negative bacteria in Plinthudult. N fertilizer and SAR, as well as the interactions between them, had a significant impact on bacteriain Plinthudult, all microbial community in Paleudalfs and total PLFAs in both soils. |
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